This invention relates to green (unfired) ceramic articles and to compositions for making the same.
In the manufacture of ceramics, the evaporation of volatile constituents from a slurry composition of inorganic materials has been accomplished heretofore. U.S. Pat. No. 2,486,410 (Howatt) describes the preparation of leather-hard sheets containing finely divided ceramic and small amounts of organic solvent-soluble or water-soluble binders. The sheets are in a leathery, flexible form only as a result of some retained solvent or water. However, as pointed out in U.S. Pat. No. 2,952,877 (Park), it is difficult to retain a sufficient amount of water in the sheets on storage to keep them moldable or to control dimensional characteristics. The above-mentioned Park patent describes the use of a mixture containing, in addition to ceramic material, (a) a substantially water-soluble polymeric organic binder (e.g., polyvinyl alcohol), (b) a substantially water-soluble compatible organic plasticizer for said binder, and (c) a relatively small amount of water to give a highly viscous paste. This mixture was extruded into slabs and dried to form flexible sheets comprising ceramic particulate material, prior to subsequent stamping and firing to vitrified products.
This latter approach has also had its problems. Careful control over the amount of water present is required to permit uniform extrusion. Careful drying conditions are required to prevent loss of plasticizer and subsequent loss of flexibility of the extruded slab. The extruded product has limited storage stability due to the loss of the relatively volatile organic plasticizer. In addition, the extruded product must be stored under low humidity conditions to prevent the dry slab from taking up more water which will lead to poor control of dimensional characteristics during subsequent processing. The high water pickup is, of course, due to the use of substantially water-soluble polymeric binders and plasticizers. A later Park patent, U.S. pat. No. 2,966,719, avoided these difficulties of the water-based system, by utilizing a volatile organic solvent system wherein the tape is formed in a casting process. This method particularly differs from Howatt in that the cast ceramic tape is made flexible by the use of a compatibly plasticized thermoplastic binder resin rather than the retention of solvent. This Park patent also prefers the inclusion of an organic solvent-soluble wetting agent.
This latter Park method has also experienced several difficulties, some of which are characteristic of an organic solvent system. These include potential environmental pollution, flammability and toxicity hazards. Also, in the use of very soft thermoplastic binder the tapes are particularly sensitive to distortion on handling or roll-up on storage since the polymers are subject to creep even under low applied stress. When an external plasticizer is used, slow volatilization of the plasticizer on extended storage leads to stiffening and embrittlement. In addition, the use of an organic-soluble thermoplastic binder system provides unfired tapes which are highly sensitive to most organic solvents.
U.S. Pat. No. 3,472,803 discloses a composition for slip casting sheets to produce ceramic articles comprising an aqueous polymer latex emulsion. The latex emulsion is an uncompounded or uncured rubber having pronounced thermoplastic properties. The slurry compositions of the present invention are readily distinguishable in that the resin components are thermosetting giving the unfired films formed therefrom improved dimensional stability.
On of the major uses of the ceramic tapes is in the electronic industry. Many electronic component manufacturers print their circuitry on the ceramic tape before firing the tapes. The solvent sensitivity of these tapes limit the choice of printing compositions which the manufacturer may use. Accordingly, it is the purpose of this invention to provide ceramic slurry compositions which can be formulated into ceramic tapes and fired to ceramic articles without the above-described difficulties of the prior art. Moreover, the ceramic tapes and articles produced by this invention are particularly adaptable for use in the electronic industry.